1. Field of the Invention
The present invention relates to a layout design of a semiconductor integrated circuit and in particular to an apparatus for optical proximity correction and a method for optical proximity correction.
2. Description of the Related Art
As semiconductor devices continue to shrink in size over time, so do the individual circuit patterns. Therefore, reproducing a designed circuit pattern on a wafer by exposing a mask has recently became difficult. In particular, corner rounding and line end foreshortening are typically observed. Such phenomena are called optical proximity effects (OPE). To avoid the OPE, an optical proximity correction (OPC) has recently spread. However, as circuit patterns continue to shrink in size, a shape formed by edge lines each of which has a size that is less than a line width of the electrical circuit pattern has appeared. Such shape cannot be corrected by the OPC.
Generally, edge lines of the circuit pattern are divided to multiple portions by identifying the vertex of the pattern prior to the OPC. However, if the divided edge lines are too small, the OPC may provide an incorrect data. Such edge lines have very little affect on the projected image. However, when the OPC is applied to the pattern data containing such edge lines, excess correction is occasionally applied to the portion around the edge lines. Consequently, data applied by the OPC provides an incorrect projection image on the wafer.
Further, an OPC module adopting the OPC method identifies the corner and the line end of the circuit pattern on the assumption that the shape of the corner or the line end is simple Therefore, in the case where a hollow or protuberances, which has a size that is less than a line width, located on the line end and the corner, it is impossible for the OPC module to identify the corner and the line end. Therefore, the OPC module applies an incorrect OPC to the circuit pattern, which decreases precision of the mask.
Such shapes may be generated during the mask data processing (MDP). Some of the shapes in the pattern is meaningless for the circuit and increases the turn around time for the OPC.